Polymer compositions comprising EVOH and/or PVOH have been commonly used in order to impart oxygen barrier properties to polymer compositions. Oxygen barrier properties reduce the oxygen permeation through said polymer composition. A reduced oxygen permeation is an important feature of polymer compositions to be used in the field of packaging materials, in particular in packaging materials used for packaging of goods prone to deterioration by the action of oxygen such as food products, pharmaceutical products, and products for diagnostic purposes.
One of the advantages of using polymers such as EVOH and/or PVOH instead of other materials imparting oxygen barrier properties such as thin metal foil or vapour-deposited layers of inorganic oxides is that polymer processing techniques such as extrusion, injection molding, film blowing etc. are available which allow to produce articles with closed, pin-hole-free surfaces. However, EVOH and PVOH suffer from the drawbacks of being water-sensitive. On the other hand, the use of thick layers in order to improve oxygen barrier properties is less favourable due to the high costs of the material and its opaque appearance. In order to maintain the oxygen barrier properties of an EVOH and/or PVOH film, it is usually necessary to coat or laminate said films with moisture-barrier materials such as polyolefins. Providing such moisture-barrier layers, i.e. manufacturing a multilayer film having an inner layer containing EVOH and/or PVOH, implies increased complexity of the manufacturing process. The individual layers of said multilayer film have to be assembled in such a manner that problems like a separation of the individual layers (delamination) during use of the film is prevented.
In order to accomplish this, it is necessary to select the polymers from which the moisture barrier layers are prepared such that no compatibility problems with EVOH and/or PVOH arises in order to avoid delamination. For this reason, polyolefins have been commonly used as moisture-barrier layers.
Furthermore, processing steps such as co-extrusion, heat-laminating etc. of the individual thin layers have to be applied which implies that more complex manufacturing equipment is required. Furthermore, providing a layer in a thickness sufficient to impart satisfactory moisture-barrier properties also implies an increase of the material costs besides technical constraints with respect to the multilayer film.
Hence, it is desirable to improve the oxygen barrier properties of polymers containing hydroxyl alkylene repeating units, in particular polymers containing hydroxyl ethylene repeating units such as EVOH or PVOH, in terms of their stability against moisture such that in comparison to commonly known barrier films the thickness of the moisture-barrier layers can be reduced. It is even more desirable to improve the oxygen barrier properties of the afore-mentioned polymers in terms of their stability against moisture to such an extent that no moisture-barrier layers are required.
US 2002/0022144 A1 and US 2004/0058178 A1 describe the concept of blocking the hydroxyl groups of an EVOH polymer in order to improve the oxygen barrier properties by reacting said hydroxyl groups with an aldehyde such that an acetyl moiety is formed. It is evident that carrying out said reaction involves additional time and efforts, in particular as the reaction is carried out by either dissolving or suspending the EVOH copolymer, said aldehyde and a catalyst in an appropriate solvent such as acetic acid, water, ethanol or a mixture thereof, which solvent has to be removed after completion of the reaction. Said procedure requires time, reaction equipment and the supply of materials such as said aldehyde and solvents. Furthermore, dissolving the polymeric material in a concentration sufficiently high in order to ensure a satisfactory reaction rate and the subsequent removal of the solvents can be cumbersome.
The object of the present invention is to improve the oxygen barrier properties of polymers containing hydroxyl alkylene repeating units, in particular polymers containing hydroxyl ethylene repeating units such as EVOH or PVOH, in a simple and efficient manner.
Said object is accomplished in the manner described herein below.